In a semiconductor circuit, a particular voltage value may sometimes need to be applied to a certain part of the semiconductor circuit, such as a specific substrate or a word line, so that the semiconductor circuit functions correctly. In some cases, the particular voltage is a relatively high voltage. Such a high voltage may be generated by a charge pump circuit, which pumps a relatively low input voltage up to a relatively high output voltage. Generally, a charge pump circuit needs to work with a clock signal having a higher voltage level than a clock signal normally employed for other parts of the semiconductor circuit. For example, if a supply voltage VDD on a power rail of a semiconductor circuit is about 1.8 V, the voltage level of a clock signal in the semiconductor circuit is also about 1.8 V. In order for the charge pump circuit to generate a voltage higher than the supply voltage VDD, a high-voltage clock signal having a voltage level of about double the supply voltage VDD, i.e., about 3.6 V, is needed.
A boost circuit may be used to “boost” the voltage of an input clock signal and generate an output high-voltage clock signal, i.e., a boosted clock signal, having a voltage level of about double the supply voltage VDD. A boost circuit may comprise multiple semiconductor devices, including field-effect transistors (FET's) such as metal-oxide-semiconductor FET's (MOSFET's). When the voltage of the input clock signal is boosted higher than VDD, that boosted high voltage of about double the supply voltage VDD may also be applied to one or more of the FET's.
Sometimes, a semiconductor circuit may need to switch between a low VDD condition and a high VDD condition, e.g., between an operation condition where VDD is about 1.8 V and an operation condition where VDD is about 3.3 V. During operation when VDD is about 3.3 V, the boosted clock signal is about 6.6 V, which may be higher than the breakdown voltage of one or more of the FET's in the boost circuit, potentially causing one or more of the FETs to break down.